Spandex is the generic name for manufactured fiber in which the fiber-forming substance is a long-chain synthetic polymer comprised of at least 85% of a segmented polyurethane. Spandex is also referred to as elastane. For the sake of convenience, and not of limitation, the present invention herein is discussed in terms of spandex, but should be construed to include all embodiments described in the following disclosure and their equivalents.
Spandex is typically prepared in two steps. First a low molecular weight difunctional polymer, such as a polymeric glycol, is allowed to react with a diisocyanate to form a mixture of isocyanate-terminated prepolymer and unreacted diisocyanate (“capped glycol”). The capped glycol is then dissolved in a suitable solvent and reacted with a difunctional chain extender and monofunctional chain terminator composition to form a polyurethaneurea polymer solution. Commercial spandex fiber is then formed from the resulting polyurethaneurea solution using conventional dry-spinning or wet-spinning techniques.
By preparing the polymer in this manner, spandex comprises so-called “hard” segments derived from the reaction between an isocyanate group on the capped glycol and the chain extender. Spandex also comprises “soft” segments derived primarily from the polymeric glycol. It is believed that the desirable elastomeric properties of spandex are due, in part, to this segmented structure.
While both ends of a chain extender, like ethylenediamine, may react with isocyanate groups from the capped glycol, in certain cases only one end of the chain extender may react. The result is a polymer having a chain extender with a primary amine at one end. The number of these “chain extender ends” (CE), expressed as the concentration of ends in milliequivalents per kilogram of polymer, can be determined by measuring the concentration of primary amine in the polymer. Primary amine content can be assayed using conventional techniques.
The number of chain extender ends can be controlled by several means, such as by varying the stoichiometry of chain extender to capped glycol. Alternatively, the number of chain extender ends can be controlled using a chain terminator, such as diethylamine (DEA). A chain terminator reacts with the capped glycol, in the same manner as a chain extender, but does not have a second reactive group. The result is a polymer with a chain terminator end rather than a chain extender end. When diethylamine is used as a chain terminator, the chain terminator end is also called a diethyl urea end (DEU).
By controlling the stoichiometry of chain extender and chain terminator to isocyanate functionalities in the capped glycol, it is possible to adjust the total number of polymer ends and, therefore, the molecular weight and intrinsic viscosity (IV) of the polymer. This is known to be an effective method of controlling the molecular weight and IV of a polyurethaneurea. See, for example, U.S. Pat. No. 3,557,044, the disclosure of which is incorporated herein by reference.
The combination of a desired number of polymer ends with a desired proportion of those being chain extender ends, is an aspect of the present invention and can be described in terms of polymer properties normally measured in the art. As stated previously, the total number of polymer ends is directly proportional to the IV. The greater the number of polymer ends, the lower the molecular weight and the lower the IV. Similarly, the number of chain extender ends is related to the quantity of primary amine in the polymer. Thus, describing aspects of the present invention in terms of desirable IV and amount of primary amine, is the equivalent to describing those aspects in terms of the desirable number of polymer ends and chain extender ends, respectively. The reader is directed to the Examples for further details.
Spandex fiber can be formed from the polyurethaneurea through fiber spinning processes such as dry spinning. In dry spinning, a polymer solution comprising a polymer and solvent is metered through spinneret orifices into a spin chamber to form a filament or filaments. Gas is passed through the chamber to evaporate the solvent to solidify the filament(s). Multiple filaments can then be coalesced into a spandex yarn. The terms “fiber” and “yarn” are used interchangeably herein when referring to spandex fiber and yarn.
Solvents used in polymer solutions should provide a homogeneous solution containing little or no gels. Solvents particularly suitable for dry spinning include N,N-dimethylacetamide (DMAc), N-methylpyrrolidinone (NMP), N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Because of safety and cost concerns DMAc is preferred and, indeed, used almost exclusively in the industry.
The productivity of dry spinning is typically described in terms of grams of yarn per spin chamber per hour and is related to winding speed, yarn and filament deniers and to the number of yarns per spin chamber. Such parameters, however, are limited by the volume and solvent used in the polymer solution and the rate of solvent evaporation through the surface of each filament. The rate of evaporation, in turn, is related to the filament denier and to the number of filaments within the spin chamber. For example, an increase in filament denier, while maintaining the total yarn denier, means a decrease in overall filament surface area and a slower rate of solvent evaporation. Winding speed must be reduced in such cases to allow sufficient time for the solvent to evaporate within the spin chamber. Also, the more filaments in a spin chamber, the larger the volume of gas and solvent vapor that must be handled. High volumes of gas induce turbulence which reduces fiber uniformity, process continuity, and productivity. Further, the volume of solvent used and its rate of evaporation from the filaments may affect the physical properties of the spandex fiber such as tenacity.
It has long been recognized that if the amount of solvent used in dry spinning could be reduced (i.e., use a polymer solution with a higher percent solids), the spinning productivity would improve because there would be less solvent to evaporate from the filaments. However, a polymer solution suitable for spandex yarn production containing a maximum of only about 37 percent solids has been possible. Over the years, attempts to prepare more concentrated polymer solutions have been commercially unsuccessful because the polyurethaneurea is insoluble in DMAc above about 37 weight percent. Polymer solutions that contain more than 37 percent solids may exist, initially, but such solutions are unstable and either quickly build viscosity until they exceed the handling capability of process equipment or form gels and become insoluble. Even in those prior cases when manufacturers were actually able to produce spandex from high-solids polymer solutions, the productivity was poor and fiber had unacceptably poor properties.
To be commercially acceptable, spandex fiber must meet certain properties recognized in the industry. While small markets may exist for spandex that does not meet these properties, such niche applications are quite limited. These properties are appreciated by those skilled in the art and include, for example for spandex at 40 denier: IV greater than 0.95 dl/g; Tenacity at least 35 g; Load Power (LP) less than 7 g; Unload Power (UP) at least 0.9 g; and a coefficient of denier variation (CDV) less than 15.
Those skilled in the art will appreciate that properties for commercially acceptable spandex will vary with denier so the above illustration of commercially acceptable spandex “at 40 denier” is not to be construed to limit the present invention either to these properties or to 40 denier spandex. The present invention includes commercially acceptable spandex of other deniers which would have the above-stated properties if they were prepared at 40 denier. Accordingly, reference herein to properties of spandex at 40 denier includes spandex of different denier which would have the recited property if prepared at 40 denier.